The present invention relates to a dock leveler with a deck lift assembly that allows a deck and extended lip to rest on and float with a trailer bed as it is unloaded, a lip extension mechanism that activates when the deck is raised and deactivates when it is lowered, and a deck assembly with a combination lip lug and header plate hinge attachment.
Dock levelers are used to transfer goods between a building and a truck trailer. Dock levers bridge the gap between the building floor to the bed of the trailer or similar carrier. Dock levelers include a frame or support structure for mounting the leveler in a pit of a loading dock. The rear end of a conventional dock leveler is hinged to the building floor. The opposite end has an extendable lip plate that pivots out and onto the trailer bed. Levelers are adapted to move from a generally horizontal position where the upper surface of the deck is flush with the surface of the building floor to a second generally inclined position to provide a ramp between the bed of the truck and the dock floor.
Dock levelers are typically actuated by springs, hydraulics or the like. U.S. Pat. No. 3,137,017 pertains to a spring actuated leveler. U.S. Pat. Nos. 4,619,008 and 4,955,923 pertain to hydraulic levelers. Other dock levers are shown and described in U.S. Pat. Nos. 3,299,456; 3,368,229; 3,530,488, 3,835,497; 3,858,264;3,877,102; 3,995,342; 4,126,909; 4,279,050; 4,328,602; 4,455,703 and 4,922,568, the disclosures of which are incorporated by reference herein.
Mechanically actuated dock levelers typically support the weight of the deck by springs. The springs are biased to propel the deck upward when a hold down device is released. An operator releases the hold down mechanism to initiate the loading cycle or to reposition the leveler when finished loading. The front lip plate pivots from a hanging or pendant position to an extended position when the ramp is rising or when the operator walks the ramp down. The lip is typically extended by an actuator and held in its extended position by another mechanism. For example, the lip can be extended by a chain attached to the lower frame that tightens as the deck reaches the top of its travel as in U.S. Pat. No. 3,137,017. The lip is held in the extended position by a latch until the dock leveler is xe2x80x9cwalked downxe2x80x9d to a proper position where the lip makes contact with the bed of the trailer. The lip is then supported by the truck, and the latch falls away.
A problem with conventional dock levelers is that the hold down device does not properly allow the deck to rise with the trailer bed as the trailer is unloaded. The hold down device typically has a brake that is allowed to slip or a float spring that compress. Unfortunately, both of these designs have inherent flaws. The slipping action of the brake-type devices causes wear. Adjustment is often required to keep the proper tension. This adjustment varies with different leveler sizes and if not done properly will either slip too easily or hold too tightly and increase wear. Eventually, breakage occurs due to the constant applied friction. The use of a float spring removes the need for adjustment by replacing the slip action with a hold down spring. The problem with float springs is that they cause a xe2x80x9cbouncing effectxe2x80x9d that allows the lip plate and deck plate to separate during loading, increasing the frequency of impacts and stress on the front hinge area where most structural failures occur. This problem increases in time as the springs fatigue, weakening its holding ability and increasing the bounce and impact stress.
Another problem with conventional dock levelers is that they tend to lose their ability to fully extend the lip. The lip extension mechanisms begins to fail due to wear and other environmental considerations, a lack of lubrication or spring fatigue. Instead of latching and holding in an outward position, the lip tends to fall back to its pendent position. The leveler remains inoperative until proper lip extension is restored by periodic preventive maintenance or adjustment of the springs. Even a short outage can be significant given that these devices typically operate in a heavy industrial context.
Other levelers replace the mechanical latch with a hydraulic damper that permits the lip to be extended freely but restricted its retraction. Even if the lip is not fully extend, the damper will retard its retraction long enough for the operator to walk the leveler down to the truck bed. Hydraulic dampers also quickly retract under high load, which can protect the lip mechanism from damage when the lip is accidentally struck by a truck that backs into the lip while still extended. Still, hydraulic dampers have two significant drawbacks. First, the damper begins to retract as soon as the load is applied, and the operator must walk the leveler down immediately. Second, the viscosity of the hydraulic fluid is sensitive to changes in temperature. In warm temperatures, the lip falls too quickly for it to come to rest on the truck bed. In cold temperatures, the lip falls too slowly when the leveler is removed from the truck.
A further problem with conventional dock levelers is that the wider, longer or thicker the lip, the harder it is to extend and hold the lip in position. A loaded spring is typically used to assist in extending the lip. Although the assist spring is loaded at all times, the available force of the assist spring is contained when the lip is in its pendent or parked position. The assist spring typically does not start to work until the dock leveler is raised and the lip has already begun to extend. This loss of effective power occurs at the start of its extension, when the assist spring is needed most. Yet, increasing the tension or force of the assist springs makes it harder to walk the unit down.
A still further problem with dock leveler design is controlling the rate the lip retracts from its extended position to its pendent position. Conventional levelers use a damper as part of the lip operation to control the rate of retraction of the lip. Different dampers are required for different lip sizes and weights. One damper may allow a heavy lip to fall too quickly, or a lightweight lip to retract too slowly. As noted above, hydraulic dampers also suffer from variations in the viscosity of the hydraulic fluid due to fluctuations in temperature.
A still further problem with dock levelers is the integrity and durability of the hinge that joins the lip plate to the deck frame. This connection is a critical part of the leveler as it must withstand concentrated stresses as the fork lift and the load it is carrying traverse from the building to the trailer, or visa versa. Conventional dock leveler designs, weld a tubular hinge to the lip plate and to the header plate. The header plate is welded to the deck plate and deck support beams. The concentrated stresses on the tubular hinge traditionally result in stress cracks in the plates and their welds. A second design uses lip plate lugs to lessen these stresses. In lieu of a header plate, cooperating lugs are also welded to the support beams and deck plate. A problem with this design is that the unsupported front edge of the deck plate is more easily bent and dished between the support beams.
The present invention is intended to solve these and other problems.
The present invention pertains to a mechanically actuated dock leveler with a mounting frame secured in a pit of a loading dock, and a deck assembly with a deck and extendable lip. A deck lift assembly biases the deck to move from a parked position to a raised position to activate a lip extension assembly. As the deck is xe2x80x9cwalked downxe2x80x9d, the lip is extended and the lip extension assembly is deactivated in a controlled manner so that the deck and lip reach an engaged position against the trailer and are kept in place by a hold down mechanism. The deck assembly has a float housing with a vertical slot for releasably engaging the lift assembly to achieve a range of float positions where the deck and lip rest on and float with the trailer as it is loaded and unloaded. The deck assembly has a durable combined lip lug and header plate hinge construction.
One advantage of the present dock leveler invention is that its integrated lifting mechanism combines an upward biased deck lift assembly with a hold down device without permanently attaching the lift assembly or hold down device to the deck or ramp. The deck assembly rests on and floats with the trailer bed as the trailer is unloaded and loaded. The up and down float action created by the trailer springs is removed from the hold down, which removes unnecessary stresses and wear and tear on the dock levelers. Breaks and other friction devices that tend to wear out are avoided, as is the bouncing effect created by the lifting springs or hold down float spring.
Another advantage of the present dock leveler is that the deck remains level during storage and use. The deck lift assembly is centered symmetrically beneath the deck assembly and pushes up against the deck assembly at a central location along its width. The hold down device also attaches to the center of the deck lift assembly along its width. This symmetrical structure eliminates twisting forces on the deck or ramp by the deck lift assembly or the hold down device. Because the hold down is an integral part of and centered within the deck lift assembly, twisting caused by the lift springs or hold down device is avoided. As a result, the deck remains level during use and during storage so that the deck is level to the floor of the dock and an overhead door can close on top of it.
A further advantage of the present dock leveler is that it cushions or controls the rate of speed the floating deck drops down to its home position or raises up to its raised position. A damper is attached directly to and between the deck and deck lift assembly. This damper cushions or controls the rate of speed that the deck drops down when it returns to its home position from a floating position, such as when a trailer is pulled away from the dock when the deck and lip are still resting on the trailer. The damper also cushions or controls the rate of speed of the deck lift assembly rises to engage the floating deck assembly when the hold down mechanism is released and the deck and lip are in a floating position resting on the trailer bed. The deck damper reduces any impact forces when either the trailer leaves while the lip is still engaged on its bed, or when the hold down is released to raise the ramp before returning the ramp to its parked position.
A still further advantage of the present dock leveler is that the rear end of the lip and the front end of the deck remain in a tight abutting engagement while the deck and lip are floating on the trailer bed. This tight abutting engagement eliminates the wear and tear caused by constant impact forces when the lip pivots and separates from the deck, and then slams back into abutting engagement with the deck.
A still further advantage of the present dock leveler invention is that it avoids the aforementioned limitations of conventional lip extension mechanisms. The lip extends when the deck is walked down by a lip extension damper that is directly linked to the lip. The lip extension damper is used to extend the lip rapidly instead of only restricting the retraction of the lip. This allows the lip damper to control lip extension and retraction. Fewer parts are needed. In addition, different size lips do not diminish the effectiveness of the lip damper.
A still further advantage of the present dock leveler is its use of a lip assist spring that activates as the ramp rises and deactivates when the ramp lowers. This allows for a much easier lip extension resulting in a less walk-down force, more efficient use of power and a less restricted lip retraction.
A still further advantage of the present dock leveler is that it provides a linkage device that engages to initiate the lip extension when the deck assembly is raised, and then disengages before the lip fully extends and before the hold down device is engaged. This ensures that the lip cannot be left in an extended position to be impacted by an incoming trailer. This also allows the lip to retract if an obstruction is present at the rear of a trailer.
A still further advantage of the dock leveler is its solid and durable attachment of the lip to the deck and deck frame. A header plate is used to support the front edge of the deck plate across its full width. This header plate is combined with a lip plate lug type hinges to reduce the concentrated stresses on the tubular hinge to provide a longer structural life for the dock leveler.
Other aspects and advantages of the invention will become apparent upon making reference to the specification, claims and drawings.